The Steps of the Priory by Sally Quilford
Aka Sally's Saga, and possibly the first of several featuring the Harcourt family and residents of the local village. This book starts soon after the first world war, and covers the period up to just after the second world war. It opens with young Becky and her boyfriend Jed leaving a baby on the steps of the Harcourts' home, the Priory. We don't know exactly whose the baby is, or why it (a boy) is being abandoned, and what keeps us reading all the way through this remarkable novel is the desire to find out. Along the way there are endless twists and turns, and as with many of Sally's books, you never guess what's going to happen. There's nothing predictable about this book, though it is eminently satisfying with some memorable characters.
Sally promises a sequel and I hope she does write one!